Restorative surgery of the optical system of the eye has progressed to a new era with the advent of microsurgical techniques which allow finer and more precise operations than were previously attempted. As a consequence, the details of physiological optics have become especially important to corneal surgery.
For example, in current refractive keratoplasty procedures, the operation alters the optical characteristic of the central part of the cornea, and therefore the surgery must be done in precisely the correct location. Otherwise, the optimum therapeutic effect is degraded. Failure to place the surgical site correctly results in impaired visual acuity or other serious symptoms. There are no appropriate instruments that accurately locate and mark a surgical reference point about which the incisions can be made. Many consider the exit point of the visual axis (the point where the axis from the fovea, to the point of fixation, pierces the corneal surface) to be the ideal reference point. However, none of the instruments and techniques presently available accurately locates that point.
In the present state of the art, there is controversy on where the actual exit point of the visual axis lies and, accordingly, on what constitutes the best methodology for locating it. Some studies regard the pupillary center as the appropriate point of exit of the visual axis. (Enoch, J. M., and Laties, A. M.: "An Analysis of Retinal Receptor Orientation, II. Predictions for Psychophysical Tests", Invest. Ophthalmol., 10:959, 1971). There are practitioners who therefore advocate locating and marking the pupillary center while viewing the pupil coaxially with the point of fixation. (Walsh, P. M. and Guyton, D. L., "Correspondence", Am. J. Ophthalmol, 97:5, 1984). In contrast, there are other practitioners who have relied on an operating microscope, having a fiberoptic probe or other light source associated with one of the two microscope objective lenses to simultaneously illuminate and view the cornea. The reflected image of the light source as it reflects from the corneal surface (a point known as the corneal reflex), as the patient fixates on the source, is thereupon marked with a needed as the reference point. (Steinberg, E. B. and Waring, G. O., " Comparison of Two Methods of Marking the Visual Axis on the Cornea During Radial Keratotomy", Am. J. Opthalmol., 96:605, 1983).
These two methods for choosing the exit point of the visual axis, which represent the state of the art, are fundamentally in error and deficient in several respects.
The present techniques which make use of the corneal front surface as a mirror to produce an image of a light source as the reference point are not well founded. The corneal reflex is located on a radius of the corneal front surface, situated perpendicular to the surface at the point of reflection. If the corneal surface at that point, is not also perpendicular to the visual axis, the axis will be displaced at the surface by refractive bending. The reflected image as seen by the practitioner will then be displaced by such a tilt of the corneal surface to a false or inaccurate point. This makes the location of the corneal reflex an unreliable indicator of the point where the visual axis intersects the corneal surface.
The argument for the pupillary center is based partly on the assumption that the Stiles-Crawford Effect (SCE) shows the maximal sensitivity lobe of the fovea to be directed at the pupillary center. In reality, however, the lobe is directed only approximately at the center and may be distant from it by 1.5 mm or more in normal eyes. Surgical operations centered on the pupil of such eyes might produce significant degradation of the desired optical effect because of the false belief that the optics of the surgery were properly centered, and that scars caused by the surgery were located sufficiently far from the visual axis to create negligible light scattering or other image distortion. The utility of the pupillary center is also questionable because the pupil is frequently not circular. It is often irregular in symmetry and lacks a stationary central point during dilation and accommodation. The pupil is often induced to dilate or constrict by the application of drugs during examination or surgery. The center of the SCE sensitivity function in some subjects receiving pilocarpine to constrict the pupil has been found to be outside of a 2mm diameter circle drawn about the center of the constricted pupil. (Reference Enoch, J.M. and Hope, G.M.: "An Analysis of Retinal Receptor Orientation, IV, Center of the Entrance Pupil and the Center of Convergence of Orientation and Directional Sensitivity", Invest. Ophthalmol., Vol. 11, No. 12, pp 1017-1021, 1972). Additionally, the plane of the pupil is placed well apart from the cornea. The parallactic shifts in the two planes make the location of a point on the cornea, while estimating the pupillary center, a task fraught with inaccuracy.